You're over thinking this. It's not an immigration issue - if anything, it would be customs, and customs s only interested in goods where duty/tax is due OR prohibited goods.
Cunard has no issues, so that's the first step taken care of. YOU go through immigration, not your luggage/contents. After clearing immigration, if you have nothing to declare, you go through the 'green'. Just make sure the cremains are securely packed and that you have the certificate from the crematory.
I've googled, and found nothing to indicate that there will be any issues upon arrival in the UK.